Ichthyosaurus (left) and Plesiosaurus (right) reptiles, 19th-century illustration. These extinct marine reptiles co-existed in the Early Jurassic Period (200 to 176 million years ago). They are examples of ichthyosaurs and plesiosaurs. The largest ichthyosaurs were over 10 metres long. The largest plesiosaurs reached up to 15 metres in length. The two were often depicted in battle. The water spouts and blowholes on the Ichthyosaurus are now known to be inaccurate. This artwork was published in an 1864 edition of 'The World Before the Deluge' by Louis Figuier.